This invention relates to display apparatus and more particularly to display apparatus which enables to make an interruption into a computer by means of a light pen even when a storage tube is used as a memory device.
Display apparatus is utilized as means for efficiently performing the transmission of the information between an electronic computer and an operator, or the so-called man-machine communication. The output information from the computer is displayed in the form of a visible picture image and the operator gives a new command or input information to the computer based on the judgement or pattern recognition ability of the operator regarding the displayed picture image. In this manner, the operator and the computer freely exchange informations at high speeds. In most cases, cathode ray tubes are used to display the picture images wherein an information train given by the computer or a signal generated by a pattern generator designated by the information train is converted into a brightness signal and a deflection voltage of the cathode ray tube thereby displaying a picture image corresponding to the information. However, the cathode ray tube has no memory capability so that it is necessary to repeatedly apply the display information to the tube. To give such repeated information by the computer, it is necessary to operate the computer for a long time. In addition, as it is necessary to install a large capacity memory device for storing the display information thus overloading the computer of a limited processing capacity. To obviate this difficulty an improved system has been proposed in which a suitable memory device is contained in the display device itself for temporary storing the display information from the computer in the memory device which is repeatedly read out for display on a cathode ray tube. Although there are many types of memory devices that can be utilized for this purpose, storage tubes (for example scan converter tubes) are generally used because of their low cost, and the capability of switching the method of scanning and scanning speed. As the means for applying input informations to the computer from the display device are used the keyboard and function key of a typewriter and a light pen. The light pen functions to detect the light when an electron beam travelling on the display surface passes beneath the light pen for designating the position of a character or pattern of the picture or for applying a new information to the computer.
Let us consider a case wherein an information from the computer is written in a storage tube so as to be stored therein temporarily. The writing into the storage tube is not performed by a raster scanning system as in a television system but a character or pattern is written in the form of vectors by utilizing a random scanning system. This is because with the raster scanning system the quantity of the information to be written and the time required for it increase thereby increasing the operating time of the computer. Reading out of the written information by a cathode ray tube is different from the writing and, it is usual to use the raster scanning system by which the entire surface is successively scanned and read out for displaying on the cathode ray tube.
Where the information train sent from the computer to the storage tube is displayed on the screen of a cathode ray tube by a raster scanning system and where the position to be processed is designated to the computer by a light pen it is very difficult to obtain a correspondence between an output of the light pen (display position on the cathode ray tube) and the information train written in the storage tube by the computer in accordance with the random scanning system. This difficulty prevents the use of the light pen in the display apparatus utilizing a storage tube.